The present invention relates to a multibeam optical system that divides a laser beam emitted from a laser source into a plurality of beams and forms a plurality of beam spots on an object surface. Particularly, the invention relates to the optical system that employs a diffractive beam-dividing element to divide a laser beam emitted from a laser source.
The multibeam optical system needs a beam-dividing element that divides a laser beam emitted from a laser source into a plurality of beams to form a plurality of beam spots on the object surface.
A conventional multibeam optical system has employed a prism-type beam splitter as the beam-dividing element, which comprises a plurality of prism blocks cemented to one another. The cemented faces of the prism blocks are provided with multi-layer coatings having the desired reflecting properties, respectively.
However, when employing a prism-type beam splitter, since each one of the multi-layer coatings can divide an incident beam only into two separate beams, the number of prism blocks corresponding to the required number of separate beams must be cemented to one another. Further, when cementing one block to another block, an angle error between two cemented face unavoidably arises. Accordingly, when a large number of separate beams are required, the deviations of the beam spots on the object surface tend to become large due to an accumulation of positional errors between the cemented prism blocks.
Recently, a diffractive beam-dividing element has become used in place of a prism-type beam splitter. Since the diffractive beam-dividing element is made of a single block that is not cemented, it does not generate any positional error even when the large number of the separate beams are required.
With employing the diffractive beam-dividing element, however, since the diffraction angle of a light beam varies depending upon the wavelength thereof, the same order diffracted beam may be separated to form a plurality of beam spots in different positions on the object surface, in case a light source emits a light beam having a plurality of peak wavelengths.
For example, an argon laser, which is used as a light source of a laser photo plotter or the like, has a plurality of peak wavelengths in the ultraviolet and visible regions. Therefore, in order to avoid the above defects, it has been required to use a filter for passing a beam component of a selected peak wavelength. Thus, the beam components of peak wavelengths other than the selected peak wavelength are cut off by the filter, which results in low energy efficiency.
Further, even if a beam emitted from a light source has a single peak wavelength, in case a peak wavelength of a beam actually emitted from a light source fluctuates or varies, a beam spot pitch on a surface to be exposed is changed.
It is therefore an object of the present invention to provide an improved multibeam optical system capable of avoiding the defect such as a separation of the same order diffracted beam or a variation of the beam spot pitch caused by the wavelength dependence of a diffractive beam-dividing element employed therein.
For the above object, according to the present invention, there is provided an improved multibeam optical system that includes a compensating optical system, which is afocal and consists of a first group and a second group, arranged at the position where beams divided by a diffractive beam-dividing element are incident thereon. The compensating optical system has a characteristic such that the angular magnification thereof is inversely proportional to the wavelength of the incident beam.
With this construction, the same order diffracted beams of the respective wavelengths diffracted by the diffractive beam-dividing element exit at the different diffraction angles, and the diffracted beams are incident on the first group of the compensating optical system. Since a diffraction angle of the diffractive beam-dividing element increases as a wavelength becomes longer, an incident angle on the compensating optical system increases as a wavelength becomes longer. On the other hand, when the angular magnification of the compensating optical system is inversely proportional to the wavelength, the ratio of an incident angle on the compensating optical system to an exit angle therefrom decreases as a wavelength becomes longer. Therefore, the angular difference among the diffracted beam caused by the wavelength dependence of the diffractive beam-dividing element can be reduced when the beams transmit the compensating optical system.
In another aspect of the invention, the following conditions (1) and (2) are satisfied to counterbalance the angular difference of the diffracted beams with the variation of the angular magnification of the compensating optical system:                               v          1                =                                                            f                1                            +                              f                2                                                    f              1                                ·                      v            DOE                                              (        1        )                                          v          2                =                              -                                                            f                  1                                +                                  f                  2                                                            f                2                                              ·                      v            DOE                                              (        2        )            
where xcexd1 is the Abbe number of the first group, f1 is the focal length of the first group, xcexd2 is the Abbe number of the second group, f2 is the focal length of the second group, and xcexdDOE is a dispersive power of the diffractive beam-dividing element, which corresponds to an Abbe number of a refractive lens.
Further, it is preferable that the compensating optical system substantially satisfies the following conditions (3), (4) and (5):                                           f            1                                v            1                          =                  -                                    f              2                                      v              2                                                          (        3        )                                          f          1                =                  f          2                                    (        4        )                                          v          1                =                              -                          v              2                                =                      2            ⁢                                          v                DOE                            .                                                          (        5        )            
Each of the first group and the second group may be an element having reflecting surfaces of a positive power on which a diffractive lens structure is formed. In such a case, the conditions (1), (2) and (5) can be satisfied without difficulty. Alternatively, each of the first and second groups may be a composite element of a positive refractive lens and a diffractive lens structure.
The first group is preferably located at a position where the distance from the diffractive beam-dividing element is equal to the focal length f1 of the first group.
The multibeam optical system of the present invention is usually applied to a multibeam scanning optical system, however it can be applied to other systems as a matter of course.